VFD as a power supply frequency converter 5

[fab1961]

Dear all,

we have a machine 70 kW designed for 400 V 50 Hz which has to be
installed on a 380 V 60 Hz system. Voltage is ok, it is within the design range, but the mechanics does not allow a change in frequency and we cannot modify the electric panel. So I' ve searched on Internet for frequency converters and I've found two types: one done with motors and one static. I've asked a price for a static one but it looked expensive to me, so I asked an offer for an inverter same power (90 kW to have a safety margin for async motors starting)and the price is less than the half.
I can understand why:
a) the inverter is manufactured on a large scale
b) the static converter has an isolation transformer at the output plus filters to give out a good sine wave, not a PWM

Am I missing something important? thank you in advance for your advice.

 

[dpc]

If this "machine" is an induction motor, a VFD should work. You don't need a sine wave output for an induction motor.

David Castor

http://www.cvoes.com

 

[jraef]

One main difference may also be inrush capability. VFDs are intended to ALWAYS ramp power into a motor (or motors), where as if you want an overall frequency converter, it's implied that loads will be connected to it that will turn on and off randomly. That is something a VFD is not designed to do and using it in that way can damage the transistors.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[krsh]

If by machine you mean electric motor the answer is yes otherwise no.

 

[sibeen]

b) the static converter has an isolation transformer at the output plus filters to give out a good sine wave, not a PWM

You probably need to look harder. I can probably think of a manufacturer or two who still make a system, in that power range, that doesn't use PWM, but it would be a struggle. The same goes for the output transformer.

Most of the UPS systems, in this power range, have the facility to take 60 HZ in and output 50 Hz. They will all (OK, nearly all) be transformer less and utilise PWM as their switching mode. They are going to be way more expensive than a standard drive but as others have stated it depends upon what your load is whether a drive is suitable or a decent sine wave output is required.

 

[LionelHutz]

You can use a standard VFD to operate the motor if you directly connect it. Do not attempt to switch the motor on and off at the output of the VFD. You need to convert the control system in the panel to run off the line power and then use the VFD just to operate the motor.

 

[waross]

What Lionel said. See if you can separate the controls from the motor loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fab1961]

Thank you all for the prompt reply. Actually the machine is a shotblasting machine for metals cleaning, and it has eleven async motors started in sequence (some of them DOL, others wye-delta and there is one driven by a VFD), some of them may be turned on and off according to machining cycle required, for instance the four motors 11 kW each which run the turbines which "shoot" the cleaning material onto the metal. So this definitely makes the VFD no more an option. On second thoughts too my comparison was not a fair one since the inverter requires a cabinet, a switch, fuses so the gap in price reduces a lot. I have considered the VFD as an option initially because I am not familiar with converters.
Thank you again

 

[jraef]

Simplest solution is to change all of those starters to VFDs, that Star-Delta would have DEFINITELY killed a VFD and may kill any kind of static frequency converter.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[purplepete]

If you do the VSD 60 Hz to 50 Hz job, get the VSD supplier to assist with its settings, V/f curve especially. Crudely, if the VSD outputs 380 volts at 60 Hz, it will output less volts at 50 Hz. This could lead to motor overloading if full HP are reuired. You will need to "tailor" the V/f settings so it outputs full voltage at the lower frequency.

 

[fab1961]

jraef, I agree with you but this machine we buy and resell, if it were one of ours I would have certainly done that way - anyway the company who would sell us the static type converter says the only shortcoming are the starting currents of the motors, so we have to oversize the converter - in the meantime I have searched for the other type of converters, the ones with the motors such as Temco in U.S. - I've also found a new type made by Piller company - I attach a document (sorry for the Italian) because I think could be interesting in the case you've never seen it before - just FYI

 

[fab1961]

I've translated (more or less) the file

 

[ScottyUK]

That machine - or a close relative - is the basis of their rotary UPS designs. Very good, expensive though!


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